The OFDM, OFDMA and SC-FDMA systems (we call them OFDM-like systems) are the most important air interface techniques for WiMAX and 3GPP LTE downlink and uplink. The advantages of these radio transmission techniques depend upon their capability to efficiently exploit frequency-diversity.
It is well known for those skilled in the art that the diversity gain is heavily dependent upon the channel environment. For example, no frequency-diversity can be realized in flat-fading channels. Moreover, narrow band systems like 1.25 MHz and 2.5 MHz usually experience relatively smaller frequency selectivity for a given channel scenario. This is due to the fact that the channel coherence bandwidth is determined by the user channel delay spread. In other cases, Rician Channel may also occur in some cases in macro-cell deployments, resulting in a non-frequency selective channel.
Therefore, the industry realizes that through artificially introducing frequency diversity into OFDM-like systems, the performance of the OFDM-like system can be improved significantly.
To be specific, the cyclic delay diversity (CDD) concept is proposed for the wireless telecommunication systems such as OFDM-type systems. References are made to FIG. 1a and FIG. 1b, which show the physical layer transmission structure for realizing CDD in time domain and frequency domain respectively. Since the processing for the signal in time domain is equivalent to that in frequency domain, to be specific, a delay τ in the time domain is equivalent to a phase shift in the frequency domain, the frequency domain scenario shown in FIG. 1b is taken as an example to illustrate the principle for CCD. After the source bit is processed by the traditional channel encoding, interleaving and modulation, the obtained four same branch of modulated symbols undergoes four kind of cyclic delay processing, i.e., the cyclic delay parameters e−j2πτ0/N, e−j2πτ1/N, e−j2πτ2/N, e−j2πτ3/N, which are determined based on the known channel environment, are used to carry out the phase shift process on the four branches of the same symbols, so as to obtain four branches of symbols after cyclic delay processing. Then, similar to the traditional OFDM-like system, after manipulations such as pilot insertion, IFFT (Inverse Fast Fourier Transform), insertion of Cyclic Prefix and frequency conversion, the signals to be sent are sent by the four transmitting antennas.
Reference are made to FIGS. 2a and 2b to describe the meaning of cyclic delay processing, wherein, for the easy of description, assume that the OFDM symbol is composed of a first part, shown by lines, and a second part, shown by dots, in FIG. 2a. Those skilled in the art can understand, the above assumption serves for the ease of the description for the meaning of cyclic delay processing, and does not serve as limitation to the OFDM symbols in the present invention.
According to general OFDM-like system, in the scenario that no cyclic delay is carried out to the signals to be processed (i.e., the modules to carry out phase shift process to the signals with cyclic delay parameters e−j2πτ0/N; e−j2πτ1/N; e−j2πτ2/N in FIG. 1b does not exist), the OFDM symbols after IFFT transform are shown as FIG. 2a. 
According to the OFDM-like transmitter shown in FIG. 1b, the signals after cyclic delay processing then undergo IFFT transform, and then the OFDM symbols shown in FIG. 2b are obtained. Different from what is shown in FIG. 2a, the second part with dots is shifted to the place prior to the first part with lines. Therefore, the frequency selectivity is introduced, and when deep fading occurs, not all the sub-carriers of the OFDM symbols are in the state of deep fading, which is advantageous to the channel encoding and decoding, thus the robustness of the system is improved.
Based on the idea of introducing frequency selectivity by cyclic delay processing, in order to obtain the Space-Time codes gain at the same time, the CDD solution can be combined with the STBC/SFBC (space-time block code/space-frequency block code). FIG. 3 shows a physical layer transmission structure in the CDD+STBC system. Wherein, the two branches of signals after space-time/space-frequency coding are respectively processed by the cyclic delay parameters e−j2πτ0/N and e−j2πτ1/N, in order to generate four branches of signals after cyclic delay processing, with two branches for each branch of the signals after the space time/space frequency process. The subsequent processes are the same as that in FIG. 1b, which are omitted for simplicity.
In multi-antenna system, the space multiplexing (SM) technique can be used to obtain gain in data rate. However, when the channel environment becomes worse, the gain obtained by the simple SM technique can not be maximized. In order to solve the problem, the SM can be combined with the above CDD solution, the specific example of which is shown in FIG. 4.
In the present invention, the multi-antenna system with solely CDD is called CDD system for simplicity, and the transmitter therein is called a CDD transmitter; the system combined CDD and STBC is called CDD+STBC system, and the transmitter therein is called CDD+STBC transmitter; the system combined CDD and SM is called CDD+SM system, and the transmitter therein is called CDD+SM transmitter. Besides, the CDD system, the CDD+STBC system and the CDD+SM system are generally called CDD-like system, and the corresponding CDD transmitter, the CDD+STBC transmitter and the CDD+SM transmitter are generally called CDD-like transmitter.
In the existing CDD-like system, the cyclic delay parameters (for example τ0 . . . τ3 shown in FIG. 1a, e−j2πτ0/N; e−j2πτ1/N; e−j2πτ2/N; e−j2πτ3/N shown in FIG. 1a, etc), which is used to carry out cyclic delay processing on the signals to be processed, are time-invariant. That is to say, take FIG. 1b as an example, the signal sent by the antenna TX_1 is the signal which is cyclic delay processed by the e−j2πτ0/N, and the signal sent by the antenna TX_2 is the signal which is cyclic delay processed by the e−j2πτ1/N (analogies can be made to other antennas, for which are omitted).
However, since CDD-like system is sensitive to inter-antenna spatial correlation and the inter-antenna spatial correlation is dependent on the distance between the antennas and some time-variant values, such as angle of arrival, angle spread, it is difficult to predefine a group of cyclic delay parameters that enables the system to always exhibit a good performance in the constantly varied wireless channel environment. Usually, in the CDD-like systems using the above time-invariant cyclic delay parameter, the system performance deteriorates with the time passes by.